Melt the dark chocolate in a heatproof bowl over simmering water, stirring until smooth.
In a separate bowl, whisk the egg whites with a pinch of salt until stiff peaks form.
Gradually add sugar to the egg whites, continuing to whisk until glossy.
In another bowl, beat the heavy cream with vanilla extract until soft peaks form.
Allow the melted chocolate to cool slightly, then stir in the egg yolks until combined.
Gently fold in the whipped cream to the chocolate mixture until no streaks remain.
Carefully fold in the beaten egg whites in three additions, being gentle to maintain volume.
Spoon the mousse into serving dishes or glasses.
Cover and refrigerate for at least 2 hours, or until set.
Garnish with chocolate shavings or mint leaves before serving.